From the New York Times:

Few Americans had ever thought of Japan as a source for puzzles until a little more than two years ago, when sudoku suddenly took the nation by storm, flooding airport gift shops, and even rivaling crosswords in popularity. Now Nikoli, which publishes puzzle magazines and books, is widely regarded as the world's most prolific wellspring of logic games and brainteasers.

Mr. Kaji and the company have had a hand in creating and promoting most of the half dozen or so number puzzles that have taken off after sudoku. But Mr. Kaji says that Nikoli has at least 250 more puzzles like sudoku, the vast majority of them unknown outside Japan.